you could make it a brackish tank... keep a figure 8 puffer and some fancy snails, as well as possibly a couple mollies. The puffer will cheerfully eat baby snails & baby mollies. Snails are important to the diet of puffers... shrimp species are also recommended, and will probably also be eaten by the puffer as it grows. Figure 8's grow to about 3" which is about perfect for your 15 gallon tank as long as you're not keeping too many other fish (puffers are very territorial and might or might not tolerate tankmates for very long). I'd try and keep some java moss for snails/shrimp/mollies/etc to munch on.
I love puffers they're awesome.
Pea puffers are adorable... they only get up to about 1" so you could try keeping a couple if your tank has plants or hiding spots.
Other than puffers... hmm...
The fancy guppies were a good idea. If you're willing to spend the money you can get some beautiful varieties for a show tank.
People do set up 15-gallon marine nano tanks, but due to stability issues associated with small tanks, this is definitely not recommended unless you have past experience with saltwater aquariums.
You could try a 15-gallon planted tank, if you find various low-light plants or purchase a quality lighting system. I'm sure many users could help you find & procure low-light species for your tank. Plants generally do not fare well in brackish water, so this would not work well in combination with mollies & figure 8 puffers.
I'm interested to hear more suggestions from other users - I'm sure we all want out tanks to be unique and satisfying, even the smaller tanks.